This invention relates to a loudness control circuit for an audio device used for compensating for loudness characteristics of the human ear.
As is well known, the human ear has loudness characteristic according to which it becomes harder for a listener to hear bass tones and high tones as a sound volume becomes smaller, even though sound-pressure (strength of sound) of the tone remains the same. For expressing the loudness characteristic specifically, the Fletcher-Munson curves shown in FIG. 6 are known. As will be apparent from these curves, when volume of a sound is relatively large (e.g., 120 phon), sound-pressure levels at which tones of low, middle and high frequencies can be heard at 120 phon are almost the same whereas when volume of a sound is relatively small (e.g., 20 phon), sound-pressure levels at which low frequency and high frequency tones can be heard at 20 phon become larger than that of a middle frequency tone.
This poses a problem in playing back music from audio devices such as a tape recorder and a Compact Disc player. Referring to FIG. 7, curves L1, L2 and L3 show the relationship between sound-pressure and level in auditory sensation with respect to tones of 2 KHz, 10 KHz and 100 Hz. Assume now that a range H1 in the figure represents a range of sound-pressure of a live music (i.e., music at recording time) and a range H2 represents a range of sound-pressure of a reproduced tone. In this case, the difference in auditory sensation (i.e., difference in auditory sensation between a tone of a middle frequency and a tone of a low frequency) is sign A at a sound-pressure level M1 which is a center sound-pressure level of the range of live music but it is enlarged to sign A' when the music is reproduced and, particularly when sound-pressure is small, difference B in auditory sensation is enlarged to B'.
There has been provided in prior art playback devices a circuit for compensating for this difference due to the auditory sensation characteristic, i.e., a loudness control circuit which boosts low frequency and high frequency tones. FIG. 8 shows an example of such a loudness control circuit. In FIG. 8, reference numeral 1 designates an input terminal for inputting the reproduced signal, 2 a variable resistor, 3 an amplifier, 4 a variable resistor, 5 and 6 capacitors, 7 and 8 resistors and 9 an output terminal respectively. A signal from the output terminal 9 is supplied to a loudspeaker (not shown). The variable resistor 2 is provided for controlling tone volume level and the variable resistor 4 is provided for establishing a ratio of boosting low frequency and high frequency tones.
The prior art loudness control circuit has the disadvantage that, since the ratio of boosting low frequency and high frequency tones is constant unless the variable resistor 4 is manipulated, low frequency and high frequency tones tend to be overemphasized when tone volume becomes large.
It is, therefore, an object of the invention to provide a loudness control circuit which is capable of performing an optimum boosting in response to tone volume of a reproduced tone.